Relatively recently infusion apparatus has been developed which can be implanted in the body and remain there for a prolonged period. The apparatus can be refilled with infusate without having to remove the apparatus from the body. This refilling is accomplished by injecting additional infusate through a penetrable septum in the apparatus which septum is located directly under the patient's skin. In some cases, the act of refilling the apparatus with infusate also recharges the apparatus' power source so that the device can operate uninterruptedly to dispense infusate such as insulin, heparin or a chemotherapeutic agent at a very small rate over a long time. Examples of infusion apparatus of this general type are disclosed in U.S. Pat. Nos. 3,951,147 and 4,258,711, both owned by the assignee of the present application.
Some infusion apparatus of this type such as the one described in the latter patent have the capability of dispensing a continuous basal dose of infusate to the patient as well as larger bolus doses when the occasion demands it. For example, a patient may receive a continuous dose of insulin based on the average glucose level in his blood. Then, right after mealtimes, when the glucose level usually rises, the pump can be made to dispense a bolus dose of insulin to compensate for that increased glucose level due to the ingestion of food. In order to provide such dual dosage capability for any reasonable length of time, the pump must have a large infusate reservoir to provide both the basal and bolus requirements or two smaller reservoirs, one of which contains the infusate at the basal concentration and the other of which contains infusate at a higher bolus concentration. In either event, the inclusion of the bolus dispensing capability increases the overall size of the pump envelope.
In some instances, however, a supplement to the basal infusate flow is required relatively infrequently. For example, it may only be introduced to add an angiographic solution to the infusate so that the areal distribution in the body being infused can be monitored from time to time. It would be desirable in such cases to provide this capability in a pump having a minimum overall size or envelope. It is also essential that this added capability be provided without adversely affecting the basal infusate contained in the apparatus or its flow to the patient.